Alfvénic Heating in the Ionosphere-Thermosphere
Abstract
The effect of electromagnetic variability in ionosphere-thermosphere heating is examined using a new method for reconstructing Alfvénic electric and magnetic fields measured on low-altitude satellites. The study is motivated by CHAMP satellite observations of correlations between thermospheric density anomalies at F-region altitudes and small-scale field-aligned currents (SSFACs), interpreted by Rother et al. (2007) as evidence of ionospheric Alfvén resonator modes. The ensemble Alfvén wave expansion described by Hedlund et al. (see abstract, this meeting) is used to disambiguate the spatial and temporal dependence of the observed SSFACs and derive heating rates from the resulting Alfvén-wave electric fields and FACs. Height-integrated and height-dependent heating rates for Alfvén waves incident from the magnetosphere at frequencies from 0.05 to 2 Hz and perpendicular wavelengths from 0.5 to 20 km have been calculated. The minimum in Alfvén speed at the F-region density maximum facilitates energy deposition by slowing, trapping and intensifying resonant waves. The Alfvénic Joule heating rate maximizes at the resulting resonances. F-region Joule heating rates resulting from quasistatic and Alfvénic variability with the same root-mean-square amplitude in the F-region are shown to be comparable. At the same time, Alfvénic variability at frequencies above 0.1 Hz deposits little electromagnetic power in the E-region whereas quasistatic electric fields greatly enhance E-region heating. When measured electric and magnetic fields are used to constrain the amplitude and spectral content of superposed Alfvén waves incident from the magnetosphere, the calculated F-region heating rate is sufficient to produce thermospheric density anomalies observed at 400 km altitude.
Rother, M., K. Schlegel, and H. Lühr (2007), CHAMP observation of intense kilometer-scale field-aligned currents, evidence for an ionospheric Alfvén resonator, Ann. Geophys. 25, https://doi.org/10.5194/angeo-25-1603-2007- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMSM43A..03L
- Keywords:
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- 2431 Ionosphere/magnetosphere interactions;
- IONOSPHEREDE: 2475 Polar cap ionosphere;
- IONOSPHEREDE: 2721 Field-aligned currents and current systems;
- MAGNETOSPHERIC PHYSICSDE: 2736 Magnetosphere/ionosphere interactions;
- MAGNETOSPHERIC PHYSICS